Molded foam products, in which many bubbles are present in the inside of resins, are used in various fields because excellent physical properties such as lightweight properties, heat insulating properties, sound absorbing properties, and rigidity on a unit mass basis, are exhibited. Particularly in recent years, weight reduction has been highly valued because the weight reduction leads directly to reduction of the raw material cost, the transportation cost, and the like. Therefore, application fields of molded foam products have been further expanded. Methods for foam-molding resins are classified into a physical foaming method and a chemical foaming method depending on the type of foaming agent to be blended into the resin. In the physical foaming method, inert gases such as nitrogen and carbon dioxide and volatile substances such as hydrocarbons and fluorocarbons, which are physical foaming agents, are used. On the other hand, in the chemical foaming method, organic foaming agents such as azo compounds and nitroso compounds and inorganic foaming agents such as sodium bicarbonate, which are chemical foaming agents, are used. In a injection foaming molding method, in which the above-described foaming method is applied to an injection molding method serving as a method for molding a resin, a foaming-agent-containing molten resin, in which a foaming agent and the resin have been mixed, is injected and filled into a mold cavity and the resin is foamed, so that an injection-molded foam product having a bubble diameter of about 80 to 300 μm in the inside of the molded product can be produced.
For the method for foam-molding a resin, for example, a molding method in which a chemical foaming agent or a physical foaming agent is blended into an olefin resin, the resulting mixture is melted, and a molded foam product including bubbles in the inside of the molded product is produced by using a short-shot method (low-pressure method), is disclosed (refer to Patent Document 1). In the method disclosed in Patent Document 1, a molding apparatus is composed of an extruder, an accumulator, and a mold. A foaming-agent-containing molten resin prepared by mixing a physical foaming agent, which is an inert gas such as a nitrogen gas; a volatile substance such as a hydrocarbon or a fluorocarbon; or the like, or a chemical foaming agent and a resin is fed into the accumulator with the extruder, the foaming-agent-containing molten resin fed into the accumulator is injected into the mold, and the resin is foamed so as to produce an injection-molded foam product including bubbles in the inside of the molded product.
On the other hand, for the method for producing a molded foam product by using a physical foaming agent, for example, a method, in which a gas such as air; a volatile substance; or the like is supplied from an extruder hopper under a pressure concurrently with supply of a resin, and melting of the resin and inclusion and dispersion of bubbles are performed with a screw extruder, is disclosed (refer to Patent Document 2). In the method disclosed in Patent Document 2, a polyethylene is used, and air is supplied at a pressure of about 0.69 to 0.78 MPa (7 to 8 kgf/cm2) so as to produce an extrusion-molded product of spongy substance including isolated bubbles.
For example, a method, in which a bubble density (the number of bubbles per unit area) is significantly increased as compared with the density of bubbles formed in the inside of a molded foam product by using a known chemical foaming agent or a physical foaming agent, is disclosed (refer to Patent Document 3) as a method in which carbon dioxide that is an inert gas serving as a blister gas is used in a supercritical state so as to produce a molded foam product. In the method disclosed in Patent Document 3, a system composed of a pressurizer for a supercritical fluid, an apparatus for supplying the supercritical fluid, and a gas bomb is attached to a molding apparatus, carbon dioxide in a supercritical state is infused from a plasticizing cylinder of the molding apparatus so as to dissolve into a molten resin, the molten resin, in which infused carbon dioxide is dissolved, is injected and filled into a mold, and the resin is foamed so as to produce a molded resin product having ultrafine microporosity of below 1 μm, which is referred to as microcell, in the inside of the molded product.    Patent Document 1: JP-A-S44-006080    Patent Document 2: JP-A-S43-009913    Patent Document 3: JP-K-H06-506724